Reactive Magnesia Cement Research Articles

Long-term mechanical performance of fly ash-incorporated reactive magnesia cements

In this research, after producing reactive magnesia directly from calcination of magnesite, reactive magnesia specimens with fly ash were prepared and cured in two different environments to investigate their long-term mechanical performance by carbonation. To investigate the effect on their microstructures of carbonation, the XRD profiles of the samples were also obtained and investigated. It is found that the strength of reactive magnesia samples is highly affected by the addition rate of reactive magnesia and the curing regime. In spite of the presence of the nesquehonite (MgCO3.3H2O) in the reactive magnesia cements having no portland cement, any type of magnesium carbonate crystal was not found for the cements with portland cement in carbon dioxide curing. The strength increase in reactive magnesia cements can be due to the incorporation of Mg2+ ions to the calcite crystals.

Investigating key factors for superior CRMC-based mortars cured under CO2 pressurized environment

An exploratory programme was undertaken to individually test the influence of reactive magnesia to waste-based material (biomass fly ash) ratio, water to solids in binder ratio, static compaction pressure, accelerated carbonation curing period, and curing temperature, on the compressive strength results of Carbonated Reactive Magnesia Cement (CRMC)-based mortars. Subsequently, the observed optimal conditions of each parameter were combined to assess their influence when applied together. A total of twenty-four mixture labels were designed, with specimens moulded under static compaction pressure and subjected to a pressurized accelerated carbonation curing under controlled conditions. The devised mortars were evaluated through compressive strength tests, and the microstructure was carried out through TG-DTG, ATR-FTIR, XRD, SEM-EDX, and MIP analyses. The results demonstrated that none of the tested influencing factors can be exclusively considered as the sole determinant for the compressive strength outcomes. Thus, this study highlights the importance of thoroughly considering any modifications in both mixture design and accelerated carbonation curing conditions to achieve the optimal properties of CRMC-based mortars. Additionally, the devised CRMC-based mortars exhibited favourable binding properties with biomass fly ashes, allowing for its secondary use and contributing to waste circularity by avoiding landfill disposal, supporting the development of the waste circularity and CO2 sequestration through mineralization in carbonated materials.

Investigation of non-uniform carbonation in strain-hardening magnesia composite (SHMC) and its impacts on fiber-matrix interface and fiber-bridging properties

Unlike Portland cement, the reactive magnesia cement (RMC) develops its strength via carbonation. Non-uniform microstructure and properties due to the varied carbonation degree across cross-section depth have thus been observed. However, the existing micromechanics design theory employs uniform material properties (cementitious matrix and fiber-matrix interface characteristics) to design strain-hardening composites. This research investigated such non-uniform carbonation in a strain-hardening magnesia composite (SHMC) and its impacts on fiber-matrix interface and fiber-bridging properties. Results showed that mineral phase composition (of the cementitious matrix) and fiber-matrix interface properties across cross-section depth are highly non-uniform in SHMC due to varied carbonation degrees. The outer layer exhibited a much higher carbonation degree with significantly stronger fiber-matrix friction, while the inner core showed negligible carbonation with distinct fiber-matrix properties. Insights gained from the micro-scale investigation were used to calculate the fiber-bridging constitutive law of SHMC with non-uniform carbonation across cross-section depth and to assess its strain-hardening performance. This investigation highlights the importance and necessity of considering non-uniform carbonation in the modeling and design of SHMC. The study presents a framework for the consideration of non-uniform micromechanical parameters in the design of strain-hardening cementitious composites.

Effect of internal moisture content (IMC) on the CO2 sequestration efficiency of hollow natural fiber (HNF)-reinforced reactive magnesia cement (RMC) composites

The incorporation of minor hollow natural fibers (HNF) can significantly accelerate the CO2 diffusion in reactive magnesia cement (RMC). The CO2 diffusivity in HNF is highly related to its internal moisture content (IMC), which subsequently affects the carbonation process of HNF-RMC, but this is outside the scope of the existing studies. Therefore, this work investigates the effect of IMC on carbonation, strength, and microstructure of HNF-RMC composites. HNF-RMC mortar specimens are dried to different levels of nominal IMC (0–100%), and cured under high CO2 concentration for carbonation. Following the curing, the matrix composition was characterized with varying techniques (QXRD, TGA, FTIR, and acid digestion) for quantifying the CO2 sequestration; the compressive strength at different depths was determined with uniaxial compressive test; the microstructure was examined with SEM/ESEM. The results show that the carbonation depth is increased by three to four times by reducing the IMC to 0%. The effects of varying IMC on local carbonation and strength are distinct in the shallow (0–20 mm) and middle/deep region (20–100 mm). In the shallow region, inadequate CO2 dissolution is the limiting factor, these performances decreased with the increasing desiccation. In the deeper regions, moisture absorption by HNF decreased the CO2 diffusivity, hence the CO2 availability is the limiting factor, and these performances increased with the desiccation. These observations suggest that the understanding that carbonation maximizes at a medium moisture level is not valid in HNF-RMC–controlling the moisture can be important for improving the CO2 sequestration and consequent mechanical strength.

Effect of hollow natural fiber (HNF) content on the CO2 diffusion, carbonation, and strength development of reactive magnesium cement (RMC)-based composites

Reactive magnesia cement (RMC) is an emerging class of green cement that hardens by sequestering CO2. However, CO2 diffusion into RMC is restricted to a few millimeters by the carbonation-induced dense microstructure on the outer layer, which severely slows down the strength growth and CO2 sequestration. To address this issue, this work employed hollow natural fibers (HNFs) to facilitate CO2 diffusion into the deep regions of RMC. The effects of HNFs contents on the mechanical strength development, holistic porosity, CO2 sequestration, CO2 diffusivity, and microstructure of RMC were investigated through different techniques. The findings revealed that the compressive strength could be more than doubled with the addition of adequate sisal fiber. Moreover, the CO2 sequestration and diffusivity could be continuously enhanced with the increasing HNFs content. However, overdosage of HNFs could induce a higher porosity and additional defects, which slightly compromises the mechanical strength. Finally, the durability of HNFs in simulated RMC and Portland cement (PC) environment was compared by accelerated aging test, showing that the alkaline-induced deterioration of HNFs could be almost eliminated in RMC. Therefore, this preliminary study reinforces the function of RMC as a carbon reservoir and lays the foundation for the large-scale utilization of HNFs in RMC.

The effect of depth-dependent carbonation on the interfacial strength between GFRP reinforcement bar and reactive magnesia cementitious composites

Reactive magnesia cement (RMC) is an emerging class of low-alkaline and CO2-sequestering binder, which can mitigate the deterioration of GFRP reinforcements induced by a high-alkaline environment, e.g., in Portland cement. This study investigated the slip behavior of GFRP rebar embedded in RMC composite, which varies with carbonation depth significantly. The variation of the interfacial bond was determined by a specially designed push-out test of the GFRP core; the variation of the carbonation degree and microstructure was examined by SEM-EDX, MIP, XRD, TGA, FTIR, and acid digestion tests. Both properties demonstrated a similar trend, decreasing rapidly with increasing depth. A new finite element model that considers the depth-dependency of the matrix compositions and the rebar-to-matrix interfacial bond is established. It can predict the constitutive bond-slip behavior of a long GFRP rebar embedded in an RMC composite with non-uniform carbonation.

Efficient stabilization of dredged sludge with high water content using an improved bio-carbonation of reactive magnesia cement method

This study proposed an improved bio-carbonation of reactive magnesia cement (RMC) method for dredged sludge stabilization using the urea pre-hydrolysis strategy. Based on unconfined compression strength (UCS), pickling-drainage, and scanning electron microscopy (SEM) tests, the effects of pre-hydrolysis duration (T), urease activity (UA) and curing age (CA) on the mechanical properties and microstructural characteristics of bio-carbonized samples were systematically investigated and analyzed. The results demonstrated that the proposed method could significantly enhance urea hydrolysis and RMC bio-carbonation to achieve efficient stabilization of dredged sludge with 80% high water content. A significant strength increment of up to about 1063.36 kPa was obtained for the bio-carbonized samples after just 7 d of curing, which was 2.64 times higher than that of the 28-day cured ordinary Portland cement-reinforced samples. Both elevated T and UA could notably increase urea utilization ratio and carbonate ion yield, but the resulting surge in supersaturation also affected the precipitation patterns of hydrated magnesia carbonates (HMCs), which weakened the cementation effect of HMCs on soil particles and further inhibited strength enhancement of bio-carbonized samples. The optimum formula was determined to be the case of T = 24 h and UA = 10 U/mL for dredged sludge stabilization. A 7-day CA was enough for bio-carbonized samples to obtain stable strength, albeit slightly affected by UA. The benefits of high efficiency and water stability presented the potential of this method in achieving dredged sludge stabilization and resource utilization. This investigation provides informative ideas and valuable insights on implementing advanced bio-geotechnical techniques to achieve efficient stabilization of soft soil, such as dredged sludge.

CO2-sequestering ability of lightweight concrete based on reactive magnesia cement and high-dosage biochar aggregate

Lightweight concrete is widely used in non-structural components of buildings, e.g., partition wall bricks. Most current lightweight concretes engage Portland cement and lightweight aggregates. In this study, lightweight concretes based on reactive magnesia cement (RMC) and high amount of biochar aggregates (up to 508 kg/m3, 61 vol% of the concrete) are developed. The new material demonstrates the ability to sequestrate massive amounts of CO2 from ambient air, and thus has the potential to be used for greener non-structural applications. The CO2 sequestration during curing under high-concentration CO2 (10 %), which is mainly via the RMC carbonation, is determined by acid digestion. The CO2 sequestration during service under ambient CO2 concentration (0.1%, or 1000 ppm), which is via the continued carbonation of RMC and the adsorption of biochar, is determined by monitoring the removal of CO2 in situ; particularly, the CO2 sequestrations by the RMC and the biochar are determined separately. The effect of varying biochar content and CO2 curing time on the overall CO2 sequestering ability of the concrete are studied, together with the variation of the local CO2 sequestration with carbonation depth in the concrete. Based on the experimental results, the new concretes are seen to achieve adequate mechanical properties for non-structural application. They can permanently sequestrate a significant amount of CO2 during the curing phase (0.2–0.3 g per gram RMC), and constantly sequestrate noticeable amounts of CO2 during the service stage (1–12 mg per gram RMC, 1 mg per gram biochar). The incorporation of porous biochar not only decreases the material density, but also improves the RMC carbonation by accelerating CO2 penetration, while the RMC barely influences the biochar adsorption. However, the RMC carbonation ability permanently vanishes with curing age, whilst the biochar adsorption decreases marginally with the curing age.

Autogenous healing of the interface between hollow natural fiber (HNF) and reactive magnesia cement (RMC) matrix

The fiber-matrix interface healing determines the restoration of tensile strength of fiber reinforced cementitious composites (FRCCs). This work first-timely investigates the autogenous healing of debonded interface between hollow natural fiber (HNF) and reactive magnesia cement (RMC) matrix and subsequent tensile strength recovery. HNF-RMC single-fiber specimens are preloaded to induce interfacial debonding, and then conditioned under water-air cycles for healing. The healing-induced recovery of the interfacial properties in the HNF-RMC group are significantly higher than the control PVA-RMC and PVA-PC groups. Tensile test of notched FRCCs demonstrated that the tensile strength of HNF-RMC can be more than doubled after cracking-healing, and its healing efficiency is around 50% higher than that of PVA-PC and PVA-RMC. Based on microstructural characterization in situ and finite element modeling, the increased healing degree is attributed to two new mechanisms related to the special porous microstructure of HNF. First, the lumens and micropores within cell of HNF can be the pathway for CO2 and water penetration, which promotes the formation of healing products (e.g., hydrated magnesium carbonates) within fiber-matrix interface and hence the recovery of chemical and frictional bonds. Second, the healing products formed within lumens caused lateral fiber expansion, and thus prominently enhanced the frictional bond.

Interpretable machine learning-based analysis of hydration and carbonation of carbonated reactive magnesia cement mixes

This study explored the influence of different input variables on the hydration and carbonation degree of carbonated reactive magnesia cement (RMC) system by employing six machine learning algorithms. These included support vector machine (SVM), particle swarm optimization-based SVM (PSO-SVM), extreme learning machine (ELM), grey wolf optimizer-based SVM (GWO-SVM), kernel extreme learning machine (KELM), and extreme gradient boosting (XGBoost). The followed approach enabled the deep learning of the relevant database to achieve parameter prediction. Two feature analysis methodologies, i.e. partial dependence plot (PDP) and SHapley Additive exPlanations (SHAP), were applied to uncover the operating laws underpinning the black box operation characteristics of machine learning models. Results revealed that GWO-SVM and XGBoost outperformed all other models in predicting the hydration and carbonation degree of the complete database set (R2 of the total database set was 0.9470/0.9775 and 0.9663/0.9727 for hydration and carbonation degree, respectively). Factors such as carbonation duration, CO2 concentration, pre-curing temperature, and w/b directly influenced the degree of hydration and carbonation. Among them, carbonation duration and CO2 concentration were two of the most influential factors, resulting in the promotion of brucite consumption and accelerating the formation of carbonation products.

Influence of nesquehonite seeds on hydration and carbonation of reactive magnesia cement

Among all carbonation products of reactive magnesia cement (RMC), nesquehonite exhibited the highest CO2 amount of sequestration per molar. Inducing the nesquehonite formation may improve the CO2 sequestration capacity of RMC. In this paper, nesquehonite seeds were introduced to promote the formation of nesquehonite in RMC. The performances of RMCs with 0%, 0.5%, 1%, 1.5% and 2% nesquehonite seeds were studied and compared with that of RMC mixed with hydromagnesite seeds. It was found that the hydration of RMC could be facilitated by nesquehonite seeds, resulting in reduced setting times. The nucleation effect of nesquehonite seeds was demonstrated, resulting in increased nesquehonite formation and carbonation degree. Nesquehonite seeds combined with hydration agent can further promote the compressive strength of RMC through forming more amounts of nesqeuhonite crystals and amorphous hydrated magnesium carbonates. Compared with the hydromagnesite seeds, the nesquehonite seeds showed a more significant increase in the CO2 sequestration capacity of RMC.

Performance of Pozzolan-Based Reactive Magnesia Cement Mixes against Sulphate Attack

Reactive magnesia cement (RMC) has gained interest due to its lower production temperatures when compared to Portland cement. In this study, the performance of pozzolan-based RMC concrete samples against sulphate attack was examined. Cube samples, after being removed from their moulds, were stored in a CO2-rich environment to gain compressive strength. Information obtained from XRD showed the formation of Mg carbonates in different forms. The use of fly ash and slag in large volumes reduces the environmental impact of concrete, but the use of these components have been found to greatly affect the formation of Mg carbonates in RMC mixes. This is mainly due to their filler effects. The coexistence of Ca- and Mg-based products was found in the slag-RMC mixes. The concrete samples based on RMC underwent mass and strength losses when stored in a MgSO4 solution for up to 12 weeks. The removal of Mg from the microstructure of these samples was confirmed using SEM analysis. The use of the most widely used pozzolans at 50% by weight of the binder greatly affected the carbonation mechanism of the RMC mixes. This finding suggests that they should be limited in the design of Mg-based products that harden under CO2-rich conditions.

Comparison of magnesium oxysulfate (MOS) cement and reactive magnesia cement (RMC) under different curing conditions: Physical properties, mechanisms, and environmental impacts

The potential of MgO cement as a low-carbon alternative is widely acknowledged. However, there has been no systematic comparison of the physical properties, mechanism and environmental impact between reactive MgO cements (RMC) and MOS Cement. This study investigated the hydration and carbonization processes of RMC and MOS cement within the range of reactive MgO/MgSO4 molar ratio from 7 to 350. Results showed that different curing conditions had different impacts on RMC and MOS. Specifically, the density of both materials exhibited an increase, but the strength of RMC was enhanced while that of MOS decreased. Although the carbonate and 517 phases can coexist in MOS specimens under carbonization curing, their combination resulted in a notable strength decrease. Carbon footprint analysis showed that RMC cement had certain advantages in carbon emissions over the entire life cycle. Overall, these research outcomes provided valuable guidance for the design of RMC and MOS cement.

Pore structures and interfacial properties between hydrated magnesia carbonates-modified recycled aggregate and reactive magnesia paste

To further increase the sustainability of reactive magnesia cement (RMC) concrete while not compromising their overall strength, this study used the carbonation technique to pretreat recycled aggregates during the preparation of RMC-based samples, and their strength, pore structures and carbonation degrees were assessed by the compression test, XRD, TG-DTG, FTIR and X-CT. Greater carbonation degrees are considered to contribute to the strength improvement due to the formation of interconnected and well-developed carbonates and reduced porosity in RMC-based samples. However, this study shows that lower w/c ratios, which greatly refined their pore structures, favor the improved mechanical performance of RMC-based samples, albeit their carbonation degrees are lower. In addition, the much weaker correlation between the carbonation degree and strength of RMC-based samples could also be explained by the formation of different types of hydrated magnesium carbonates (HMCs), where samples containing nesquehonite (∼needle-like HMCs), albeit their carbonation degrees could be lower, reveal better strength than samples with hydromagnesite (∼rosset-like HMCs). Therefore, using pretreated recycled aggregates in RMC-based samples delayed their inside CO2 diffusion when they were cured in CO2, which slowed down their strength development, but when cured in air, RMC-based samples containing pretreated recycled aggregates reveal better strength owing to their more compacted pore structures. This study could also prove that CO2 diffusion among RMC-based samples could be through the pores within ITZs instead of capillary pores resulting from water.

A novel bio-carbonation method of reactive magnesia with urea pre-hydrolysis for geomaterial stabilisation

This study proposes a novel bio-carbonation method of reactive magnesia cement (RMC) with urea pre-hydrolysis for geomaterial stabilisation. Urea pre-hydrolysis in the bacterial solution before mixing with RMC produces more carbonate and ammonia, facilitating the RMC bio-carbonation to form more hydrated magnesia carbonates (HMCs), the dominant product that contributes to strength gain. The effect of urea pre-hydrolysis on the engineering performance of bio-carbonated samples is investigated through a series of RMC bio-carbonation geomaterial stabilisation tests with various pre-hydrolysis times and urea concentrations. The results show that the proposed method can improve material utilisation, enhance the RMC bio-carbonation reaction and promote geomaterial stabilisation performance. The relationships between the unconfined compressive strength (UCS) of bio-carbonated samples and the amount of HMCs are studied using a proposed indicator, the degree of carbonation (DC). However, the strength increment is not monotonically related to the amount of HMCs. The optimal pre-hydrolysis time is 6–12 h for bio-carbonated samples with UCS up to 3·79 MPa, and the maximum UCS increase in the samples is up to 404·42%. The critical DC values are in the range 0·23–0·36 with varying urea concentrations. The proposed method has implications for the design of bio-carbonated RMC in field applications.

Recovery of ultra-high purity reactive magnesia from reject brine and its comparison with commercial magnesia

CO2 emissions and energy consumption associated with ordinary Portland cement production have increased the urgency for the development of alternative construction materials. Reactive magnesia cement (RMC) has been widely studied in the recent years due to the lower calcination temperature used during its production and its ability to permanently sequester CO2. In addition to the calcination of magnesite, reactive MgO, the main component of RMC, can also be synthesized from reject brine, which is rich in Mg2+. However, the presence of other components such as calcium in reject brine leads to the precipitation of other mineral phases together with Mg(OH)2, thereby reducing the purity of the final product. With the goal of improving the purity and yield of MgO synthesized at the end of this process, this study proposed a selective precipitation approach under a controlled pH. An optimum condition was determined for the synthesis of magnesium oxalate dihydrate with an ultra-high purity of 99.6 % and a high magnesium recovery rate of 94.1 %. Reactive MgO with a specific surface area of 30.2 m2/g was obtained after the calcination of the synthesized magnesium oxalate dihydrate at 700 °C for 2 h, which was higher than some of the commercially available MgO powders.

Investigation of the viscoelastic evolution of reactive magnesia cement pastes with accelerated hydration mechanisms

Viscoelasticity of reactive magnesia cement (RMC) pastes containing 3 different hydration agents (HCl, Mg(CH3COO)2 and MgCl2) were investigated. Amplitude sweep, frequency sweep and time sweep of RMC pastes were examined within 3 h of hydration. Time-dependent evolution of storage modulus, loss modulus, phase angle, and shear stress were recorded. Measurements of pH, isothermal calorimetry, XRD, TG-DTG and FTIR were used to analyze hydration reaction and products. Addition of hydration agents (HAs) accelerated the growth rate of storage modulus/loss modulus over time. MgCl2 demonstrated the greatest acceleration influence, also reflected in non-destructive structural build-up and buildability related to 3D printing applications. Addition of MgCl2 and HCl advanced the initial setting time of RMC pastes to 100–110 min, during which yield stress reached maximum, and decreased afterwards. Within 3 h of hydration, pastes containing MgCl2 revealed lowest pH, highest heat release and brucite concentration. HAs inclusion precipitated brucite away from MgO particles in the bulk solution, creating a bridge between MgO particles and enabling denser microscopic network structure.

Effect of Low Nesquehonite Addition on the Hydration Product and Pore Structure of Reactive Magnesia Paste.

Reactive magnesia cement is considered an eco-efficient binder due to its low synthesis temperature and CO2 absorption properties. However, the hydration of pure MgO-H2O mixtures cannot produce strong Mg(OH)2 pastes. In this study, nesquehonite (Nes, MgCO3·3H2O) was added to the MgO-H2O system to improve its strength properties, and its hydration products and pore structure were analyzed. The experimental results showed that the hydration product changed from small plate-like Mg(OH)2 crystals to interlaced sheet-like crystals after the addition of a small amount of Nes. The porosity increased from 36.3% to 64.6%, and the total pore surface area increased from 4.6 to 118.5 m2/g. At the same time, most of the pores decreased in size from the micron scale to the nanometer scale, which indicated that Nes had a positive effect on improving the pore structure and enhancing the compressive strength. Combined with an X-ray diffractometer (XRD), a Fourier transform infrared spectrometer (FTIR), and a simultaneous thermal analyzer (TG/DSC), the hydration product of the sample after Nes addition could be described as xMgCO3·Mg(OH)2·yH2O. When Nes was added at 7.87 and 14.35 wt%, the x-values in the chemical formula of the hydration products were 0.025 and 0.048, respectively. These small x-values resulted in lattice and property parameters of the hydration products that were similar to those of Mg(OH)2.

Unlocking the role of recycled aggregates in the performance enhancement and CO2 capture of reactive magnesia cement formulations

The use of recycled aggregates (RCAs) could further enhance the sustainability of reactive magnesia cement (RMC) formulations, but the performances of RMC-RCA are still unknown to researchers. Therefore, in this study, the strength development of CO2-cured RMC-RCA was explored for the first time, and the strength variation among groups was further interpreted by microstructural analyses. The initial results show that compared with samples with natural aggregates (NAs), a 28 d strength boost (i.e., 37 vs. 15 MPa) in RMC-RCA can be observed. The strength improved in RMC-RCA could be related to its refined microstructures, ITZs and larger carbonation depths, which resulted from the easiness of CO2 penetration within samples. Then the presence of porous ITZs in RCA contributes to better CO2 diffusion, as described in the proposed revised CO2 diffusion model. Overall, while enabling a pronounced CO2 capture potential of RMC concrete, this preliminary study could pave the path for the upcycling of RCAs in RMC concrete on a large scale.

Understanding the role of seeds in reactive magnesia cement (RMC) formulations

AbstractNucleation effects of seeds in reactive magnesia cement (RMC) systems have been reported, but this process has never been directly observed. In this study, different types and contents of seeds were introduced in RMC formulations. Results show that seeded RMC samples revealed accelerated hydration/carbonation that led to their more rapid strength development, and the use of magnesite outperformed other seeds given their overall performances, as explained by their more homogenous microstructures. Further, in the model solution, the amount of Mg2+ ions adsorbed onto seeds particles continuously increased with the increment of Mg2+ concentrations, confirming the nucleation effects of seeds in RMC formulations directly, and seeds surfaces revealed a much higher affinity for Mg2+ ions than other ions due to their acid‐base interactions. Therefore, the use of seeds could act as accelerators that shorten the curing period of RMC‐based concrete products on a large scale.